1. Field of the Invention
The present invention relates to a soft start circuit and a driving method thereof, and more particularly, to a soft start circuit and a driving method thereof which utilize a soft start voltage to correspondingly generate a sink current for soft start operation.
2. Description of the Prior Art
Electronic devices usually have different elements which operate with different operational voltages. Thus, it is necessary to utilize different DC-DC voltage converters in order to achieve different voltage modulations, such as modulation for raising voltage values or dropping voltage values, and to maintain them at predetermined voltage values. Many types of DC-DC voltage converters have been widely developed and are derived from the buck/step down converter or the boost/step up converter. The buck converter can decrease an input DC voltage to a default voltage level, and the boost converter can increase the input DC voltage to another default voltage level. With development, both the buck and boost converters are varied and modified to conform to different system architectures and requirements. Also, when relatively high sensitivity of load voltage changes is required, a user will choose an application circuit, which has a feedback voltage close to a load voltage, to dynamically output the load voltage for following operation.
Please refer to FIG. 1, which illustrates a conventional schematic diagram of an error amplifier 10. Since the error amplifier 10 utilizes a feedback voltage VFB close to an output voltage (not shown in figure), the error amplifier 10 has no choice but to utilize the N-type MOS transistor as an input stage circuit. As shown in FIG. 1, the error amplifier 10 applied to a voltage converter includes a first current mirror 100, a second current mirror 102, a first switch 104, a second switch 106, a current source 108 and a third current mirror 110. The first current mirror 100 includes P-type MOS transistors MP1, MP2. The second current mirror 102 includes P-type MOS transistors MP3, MP4. The first switch 104 and the second switch 106 are realized via N-type MOS transistors MN1, MN2. The third current mirror 110 includes N-type MOS transistors MN3, MN4. Sources of the transistors MP1, MP2, MP3 and MP4 receive an input voltage VIN. Gates of the transistors MP1, MP2 and a drain of the transistor MP2 are coupled to each other. Gates of the transistors MP3, MP4 and a drain of the transistor MP3 are coupled to each other. A drain of transistor MP1 is coupled to a drain of the transistor MN3 and gates of transistor MN3, MN4. A drain of the transistor MP2 is coupled to a drain of the transistor MN1. A drain of the transistor MP3 is coupled to a drain of the transistor MN2. A drain of the transistor MP4 is coupled to a drain of the transistor MN4 to output a control signal EO. Sources of the transistors MN1, MN2 are coupled to one end of the current source 108, and another end of the current source 108 is grounded. Sources of the transistors MN3 and MN4 are grounded. A gate of the transistor MN1 receives the feedback voltage VFB, and a gate of the transistor MN2 receives a reference voltage VREF.
In other words, the prior art utilizes the feedback voltage VFB and the reference voltage VREF to correspondingly switch on the first switch 104 and the second switch 106. Under such circumstances, the first current mirror 100, the second current mirror 102 and the third current mirror 110 transform a difference between the feedback voltage VFB and the reference voltage VREF into the control signal EO to provide to a following application circuit (not shown in figure). However, when the error amplifier 10 just initiates to have a larger difference between the feedback voltage VFB and the reference voltage VREF, generation of the control signal EO accompanies an in-rush current, which can possibly cause damage of the following application circuit for receiving the control signal EO. Please refer to FIG. 2, which illustrates a schematic diagram of generation of an output voltage VOUT and an output current IOUT of the following application circuit according to the corresponding control signal EO. As shown in FIG. 2, the output current IOUT is demonstrated as an oscillation current form while the error amplifier 10 just initiates and the output voltage VOUT increases accompanying larger in-rush currents, wherein the in-rush currents are marked with dotted circle. The output voltage VOUT has a threshold, which is correspondingly limited by values of the reference voltage VREF, so as to confine product application of the error amplifier 10. Also, the conventional soft start driver circuit can not be qualified if the N-type MOS transistor is utilized as the input stage circuit.
Therefore, it has become an important issue to provide another effective control circuit which avoids the in-rush current generation of the following application circuit, so as to provide a protection mechanism to the conventional soft start circuit.